Département de chimie moléculaire

We recently reported a very efficient homogeneous system for visible-light driven hydrogen prodn. in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2]+ (1) (CR = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [RuII(bpy)3]2+ (Ru) as photosensitizer and ascorbate/ascorbic acid (HA-/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover nos. (TONs) (up to 1000) at pH 4.0 at a relative high concn. (0.1 mM) generating a large amt. of hydrogen and having a long term stability. A similar activity was obsd. for the aquo deriv. [CoIII(CR)(H2O)2]3+ (2) due to substitution of chloro ligands by water mol. in water. In this work, the geometry and electronic structures of 2 and its analog [ZnII(CR)Cl]+ (3) deriv. contg. the redox innocent Zn(II) metal ion have been investigated by DFT calcns. under various oxidn. states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concn. of the different components on the H2-evolving activity. Turnover nos. vs. catalyst (TONCat) were found to be dependent on the catalyst concn. with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel deriv., [NiII(CR)Cl2] (4), when tested under the same exptl. conditions was found to be fully inactive for H2 prodn. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA-/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq = 2.5 × 107 M-1 s-1) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket = 1.4 × 109 M-1 s-1). The reduced catalyst can then enter into the cycle of hydrogen evolution. [on SciFinder(R)]